Porous Gold Nanostructures Perform Light Tricks

The optical properties of a newly developed nanoporous gold material with long-range periodicity may generate several photonic applications. This material is prepared by packing colloidal gold particles (15 to 25 nm in diameter) between closely packed polystyrene latex microspheres (300 nm to 1 µm in diameter), then removing the microspheres.

In 1997, a team at the University of Delaware's Center for Molecular and Engineering Thermodynamics pioneered the procedure by creating similar structures from silica. The group reported its latest achievement in the Oct. 7, 1999, issue of Nature.

The porous gold material has potential for catalysis or electro-optic devices. "These gold structures act as 3-D diffraction gratings that split the reflected incident light into different wavelengths at different angles," explained Orlin D. Velev, principal investigator. "However, the greatest interest in such structures is due to their photonic crystal properties -- the ability to reflect or transmit any wavelength of light under certain conditions; and, in specific configurations, that can act as filters, resonators or waveguides."

The lacelike structure created by this procedure is almost identical to the structure of a three-dimensional wire-mesh photonic crystal that demonstrates an interesting combination of forbidden bands in the gigahertz region. But the new gold structure is 20,000 times smaller, which may result in analogous properties in the IR/visible region.

Flakes of the porous material demonstrate a combination of optical features that appear to make these structures unique among materials having periodicity on a colloidal scale. There is no special coloring or reflection at some angles; bright, pure colors are diffracted at other angles; and strong, polychromatic reflection occurs close to the incident angle.

These properties could be useful for future photonic devices, including light-based computers, Velev said. "But our goal at this stage is much more restricted. We want to understand and control the principles, to make new materials by simple methods based on self-assembly from solution. We are now creating these metallic structures in this way, and it is very encouraging." New applications might include substrates for surface-enhanced Raman spectroscopy, he added.